Electrostatic phonograph pick-up



Get; 22, 1929. H. ANDREWES ELECTROSTATIC PHONOGRAPH PICK-UP Filed April6, 1928 2 Sheet-S-Sheet 1 U m 2 U 0 w a: 22 2 (III.

Oct. 22, 1929. H. ANDREWES 1,732,393

ELECTROSTA'I'I C PHONOGRAPH PICKUP Filed April 6, 1928 2 SheetsShe-et 2Patented Oct. 22, 1929 UNITED STATES PATENT OFFICE HUMFREY ANDREWES, OFHIGHGATE, LONDON, ENGLAND, ASSIGNOR TO RADIO PATENTS CORPORATION, OFBRONX BOULEVARD, NEW YORK, N. Y.

ELEGTBOSTATIC PHONOGRAPH PICK-U]? Application filed April 6, 1928,Serial No. 268,006, and in Great Britain July 22, 1927.

In the application of electrical, or thermionic amplifying devices tothe reproduction of sounds from a gramophone record or the like, byelectrical means in lieu of the more usual direct mechanical soundreproduction, some form of pick-up device is necessary for thepreliminary translation of the sound record into electrical currents.The object of my invention is to provide an improved form of suchpick-up device, which has material advantages over such devicesheretofore known.

In carrying my invention into effect I construet an electrostaticpick-up device whereby the movements of the stylus produced by thegramophone sound record are adapted to cause variations of the capacityof an electrical condenser. These capacity variations are then caused toinfluence an electrical circuit whereby the potential of the grid orgrids of a therminoic amplifying valve are caused to vary strictly inaccordance with the details of the original sound record.

In the accompanying drawings Fig. 1 is a sectional elevation of apick-up device constructed in accordance with my invention;

Fig. 2 is a circuit diagram embodying one method of employing the saidpick-up device of Fig. l in conjunction with a three-electrodethermionic amplifying valve;

Fig. 3 is a further circuit diagram embodying the same arrangement ofpickup in Fig. 1 used in conjunction with a four-electrode thermionicvalve amplifier.

Fig. 4 is a circuit diagram embodying a method of amplification in whichtwo threeelertrode ti :rmionic valves are employed in conjunction with aradio frequency WVheatstone bridge connection to the pick-up device; and

Fig. 5 is a circuit diagram diflering from Fig. l by having onefour-electrode valve substituted for the two three-electrode Valves.

Referring to the drawings, reference numeral 1 designates the casing ofthe pick-up device which is preferably constructed in the form of ametal casting, but may if preferred be of moulded insulating material.Through the centre of this casing a bushing 2 of insulating material isfitted to carry the screw threaded rod 3 on which is the securing tionof the rod 3 and of the disc 5 attached thereto with respect to thecasing, a greater or lesser degree of pressure can be applied betweenthe said disc 5 and the flexible diaphragm 8, this pressure beingtransmitted through the insulating washer 7. The diaphragm 8 may be ofthin metal or be in the form of a diaphragm of mica or similar materialprovided with a conducting coating. At the point 9 at or near the centreof the diaphragm 8 the lever 10 is rigidly attached, this lever beingpivoted to the casing 1 by any well-known means such as is normallyemployed in the sound reproducer commonly used in gramophones. Thestylus 12 secured in a suitable holder 13 to the lower end of the lever10 is adapted to rest on the surface of the gramophone sound record andto be actuated thereby in the well-known manner. Electrical connectionsto this pick-up device 30, 31 are arranged one to the rod 3 and theother to the casing 1. If this casing is of conducting material the rod8 is insulated therefrom by means of the bushing 2, or alternatively ifthe casing 1 is of insulating material the bushing 52 may be metallic innature to provide a suitable bearing for the rod 3.

By adjusting the position of the rod 3 with respect to the casing l agreater or lesser degree of pressure can be applied to the diaphragm 8thus tensioning the diaphragm 8 and thereby controlling its naturalperiod of vibration. The faithfulness of reproduction of the sound fromthe record is considerably influenced by the natural period of thediaphragm, and it is desirable that this should be given as high a valueas possible with respect to the normal frequencies which it is intendedto reproduce. The casing of this device 1 is provided with a tubularextension 15 by means of which it is adapted for attachment to thetone-arm of an ordinary ty e. of gramophone or similar apparatus.

l n the use of this device the stylus 12 rests in the indented groove ofthe gramophone sound record and by the fluctuations in this groove iscaused to produce movements in the centre of the diaphragm 8 and therebyvary the electrostatic capacity between this diaphragm and the fixedrigid conducting plate 5. The perforations 6 in this plate provide ameans whereby the air which is trap ed in the space between the plate 5and the iaphragm 8 can move in and out of this space and prevent unduedamping of the movements of the diaphragm.

The variations of electrostatic capacity of a pick-up device constructedin accordance with Fig. 1 may be caused to vary the potential of thegrid electrode of a thermionic amplifying valve 17, Fig. 2, byconnecting the said device, which is indicated as 16 in this diagrambetween the anode and the grid electrodes of the said thermionic valve.The two electrodes of the pickup device become char ed from the hightension source 27 whic is applied to this amplifying valve, the positiveconnection being made through the resistance or impedance 19 which isconnected in the anode circuit of this valve, the negative connectionbeing made through the resistance or impedance 22 which is connectedbetween the grid and the negative terminal of the H. T. source 27. Thefluctuations of the capacity of the pick-up device 16 cause fluctuatingcurrents to flow into and out of this device and so modify the potentialof the id of the valve 17. Corresponding changes in the anode potentialof this valve are thereby produced and these may be further amplified bymeans of a second valve 18, in a wellknown manner, such as for exampleby making use of the coupling condenser 20 with grid leak resistance 21and grid bias battery 28 whereby the potential variations of the anodeof the valve 17 may be passed on to the grid of the valve 18 therebygiving rise to further amplified fluctuations of the current in theanode circuit of the said valve 18. These amplified currents may bepassed throu h the windings of a loud speaker 26 where y they may bereproduced as sounds corresponding to the details of the record to whichthe pick-up device is applied. The filaments of the thermionicamplifying valves 17 and 18 may be heated by means of current derivedfrom the battery 25 with appropriate fixed or variable series resistance23 and 24 in accordance with well-known methods.

A somewhat increased amplification of the potential variationsoccasioned by the action of the pick-up device 16 may be obtained by theuse of a four-electrode amplifying valve 29 in the manner indicated bythe diagram Fig. 3. By applying the pick-up between the two gridelectrodes of this valve 29 a greater fluctuation of the potential ofthis valve may be obtained, which fluctuations may be further amplifiedby the second valve 18 in an exactly analogous manner to that describedabove in connection with Fig. 2. Alternatively the said pick-up devicemay be connected between the outer grid electrode and the positive poleof the H. T. source 27, whereby the polarizing voltage subjected to thesaid Cpick-up device is not reduced by the voltage rep in the impedance19. The grid leak resistance or impedance 22 is referably connectedbetween the outer gri electrode of the valve 29 and the filamentnegative terminal of the same amplifyin valve. The inner grid electrodeof this va ve is preferably connected to the anode of the same valve, sothat the positive potential applied to that inner grid elecrode servesto accelerate the electron stream from the cathode of this valve andthereby render the outer grid electrode more sensitive to potentialvariations occasioned by the action of the pick-up device 16 since thisgrid is then situated in a very weak electrostatic field between theinner grid electrode and the anode of this valve. The addition of theimpedance 22 between the grid and the filament ne ative terminals of thevalve 17 and 29in igs. 2 and 3 serves the purpose of locating thepotential of these electrodes with respect to the cathode of the valve.In order to prevent this resistance dissipating too great a proportionof the potential fluctuations occasioned by the action of the pick-updevice 16 this resistance 22 may preferably be replaced by an inductiveresistance or choke coil with or without an iron core, so that thestatic potential of this id electrode can be held while the elecrode isstill free to be actuated by rapidly varying potentials produced by thepick-up device.

Any alternative means of further amplifying the potential variationsoccurring in the first amplifying stage 17 or 29 may be employed withoutdeparting from the general spirit of the invention, and where required afurther number of amplifying stages than that shown in the diagram maybe employed when required.

In carrying my invention into effect I utilize a radio frequencyWheatstone bridge as shown in Fig. 4, which consists of the ratio arms41 and 42, the adjustable arm 43, and the electrostatic pick-up device16. The bridge is fed from a valve oscillator by means of the radiofrequency transformer 45. The oscillatin valve 46 is connected to thesecondary 47 o the transformer 45 using the well-known Hartley circuittuned by the variable condenser 48 and the anode connection is madethrough the fixed condenser 49 with the usual radio frequency choke coil50 and high tension batter 27. The low tension battery 52 supplies t 1efilament current .llli

for the oscillatin valve 46 and the detector valve 60, the gri andfilament of which are connected across the galvanometer arm of theWheatstone bridge. The resistance 53 maintains the grid of the detectorvalve at a sensibly zero potential with regard to the filament when thebridge is balanced. The anode of the detector valve 60 is connectedthrough an output transformer 54 to the high tension battery 27 inwell-known manner.

The action of the circuit is as follows When the capacity of theelectrostatic pickup is caused to vary by sound waves or the like, thebalance of the VVheatstone bridge is upset thereby causing a radiofrequency difference of potential to occur across the grid and filamentof the detector valve 60 and causing a corresponding change of currentin the said valve and producing an audio frequency current in thesecondary of the transformer 54, exactly similar in form to that of thesound waves which produce the variations of capacity of theelectrostatic pick-up 16. As the Wheatstone bridge is fed from a radiofrequency source the distortion introduced in translating the soundwaves into audio frequency currents is materially reduced.

In the preferred construction of the Vtheatstone bridge, the ratio arms41 and 42 consist of condensers as shown in Fig. 4, but it is pointedout that these may be replaced by any other form of reactance withoutdeparting from the spirit of the invention. The said ratio arms 41 and42 may also be of unequal value in order to increase the sensitivity ofthe device, as the variations of the capacity of the electrostaticpick-up device will necessarily be small.

Another form of the same circuit is shown in Fig. 5 in which afour-electrode valve 55 is used to replace the two three-electrodevalves as shown in Fig. 4. The well-known oscillator circuit is employedin which the inner grid 56 of the four-electrode valve 55 is maintainedat a positive potential with respect to the outer grid 57. The radiofrequency Wheatstone bridge 41, 42, 43, and 16 is fed in similar mannerby means of the radio frequency transformer 45 in which the secondary 47is tuned by the variable condenser 48 in similar manner tothat of Fig.4. The outer grid 57 and filament 58 are then connected across thegalvanometer arm, the resistor 53 being used to maintain the normalpotential of the outer grid 57 as previously described. An audiofrequency transformer .54 is connected between the anode 59 and thepositive side of the high potential battery 27 as before.

The action of this form of the circuit is exactly similar to that ofFig. 4, wherein the variations of capacity of the electrostatic pick-updevice produce audio frequency currents in the secondary of thetransformer 54. The ratio arms 41 and 42 of the Wheatstone bridge maylikewise be replaced by other forms of reactances and may be of unequalvalue.

It will be understood that the invention described may be applied to anyother form of condenser microphone than that as referred to in thedrawings, whereby the radio frequency output currents from the secondaryof the transformer 54 may be used to influence in any well-known mannerthe modulating valve of a radio transmitter or may be ,further amplifiedand a plied to a loud speaker or :imilar device or direct speechamplifica- Having described my invention, what I believe to be new anddesire to secure and protect by Letters Patent of the United States1s 1. Means for transforming mechanical oscillations ofa phonographstylus into corresponding electrical oscillations comprising incombination a Wheatstone bridge circuit, an

electrostatic device subjected to said mechanical oscillations to changeits capacity correspondingly, said device forming one arm of saidWheatstone bridge circuit, the adjacent arm of said bridge comprising abalancing condenser, two ratio arms consisting of electrical impedancesfor completing said bridge, whereby the fluctuations of capacity of saiddevice vary the balance condition of said bridge in accordance with saidmechanical oscillations, means to impress currents to be modulated toone of the diagonal branches of said bridge and an output utilizationcircuit connected to the remaining diagonal branch of said bridgecarrying electrical currents varying in accordance with said balancevariations.

2. Means for transforming mechanical oscillations of a phonograph stylusinto corresponding electrical oscillations comprising in combination aWheatstone bridge circuit, an electrostatic device subjected to saidmechanica-l oscillations to change its capacity correspondingly, saiddevice forming one arm of said bridge comprising a balancing impedance,two ratio arms consisting of electrical impedances for completing saidbridge. whereby the fluctuations of capacity of said device vary thebalance condition of said bridge in accordance with said mechanicaloscillations, means to impress currents to be modulated to one of thediagonal branches of said bridge, a space charge controlled devicehaving main electrodes and control electrodes, means to subject saidcontrol electrode to potential variations occurring at the end of theremaining diagonal branch of said bridge and an output utilizationcircuit connected to said main electrodes of said space charge devicecarrying electrical currents varying in accordance with said balancevariations.

3. In combination, a lVheatstone bridge. an electrostatic pick up devicefor phonographs and the like, the capacity of which varies in accordancewith the mechanical oscillations of the phonograph needle, said deviceforming one arm of said Wheatstone bridge, the adjacent arm of saidbridge comprising a balancing impedance, two ratio arms consisting ofimpedances completing said bridge for maintaining said bridge normallybalanced, said device unbalancing said bridge in accordance with theoscillations of said needle and a source of high frequency connected tosaid bridge whereby said high frequency is modulated in accordance withthe unbalancing of said bridge.

4. Means for transforming mechanical oscillations into correspondingelectrical oscillations comprising a Wheatstone bridge circuit, anelectrostatic device subjected to said mechanical oscillations to changeits capacity correspondingly, said device forming one arm of saidWheatstone bridge circuit, the adjacent arm of said bridge comprising animpedance of adjustable value adapted for balancing the bridge, tworatio arms also consisting of electrical impedance for completing saidbridge, whereby the fluctuations of capacity of said device vary thebalance condition of said bridge in accordance with said mechanicaloscillations, means to produce an alternating carrier current to beimpressed on one of the diagonal branches of said bridge for beingmodulated in accordance with said balance variations and an outputcircuit including rectifying means connected to the remaining diagonalbranch of said bridge for utilizing electrical currents varying inaccordance with the said balance variations.

5. In an electrostatic pick-up arrangement for phonographs and the like,comprising a Wheatstone bridge circuit, an electrostatic pick-up deviceadapted to be subjected to mechanical oscillations to change itscapacity correspondingly, said device forming one arm of said Wheatstonebridge circuit, the adjacent arm of said bridge comprising a balancingcondenser, two ratio arms consisting of electrical impedances completingsaid bridge, whereby the fluctuations of capacity of said pick-up devicevary the balance condition of said bridge in accordance with saidmechanical oscillations, means to impress currents to be modulated toone of the diagonal branches of said bridge and an output circuitconnected to the remaining diagonal branch of said bridge for utilizingsaid modulated currents.

6. In an electrostatic pick-up arrangement for phonographs and the like,comprising a Wheatstone bridge circuit, an electrostatic pickup deviceadapted to be subjected to mechanical oscillations to change itscapacity correspondingly, said device forming one arm of said Wheatstonebridge circuit, the adjacent arm of said bridge comprising a balancingcondenser, two ratio arms consisting of electrical im edances completingsai bridge, whereby t e fluctuations of capacity of said pick-u devicevary the balance condition of said ridge in accordance with saidmechanical oscillations, means to im ress a potential difference acrosstwo diagonal points of said bridge, a space charge control device havingmain and control electrodes, said control electrode being subjected topotential variations occurring at the ends of the remaining diagonalbranch ofsaid bridge due to said balance variations, and an outpututilization circuit connected to said main electrodes of said spacecharge device for utilizing electrical currents varying in accordancewith said balance variations.

7. In combination, a Wheatstone brid e, circuit, an electrostaticpick-up device or phonographs and the like, comprising a condenserpick-up device adapted to' be subjected to mechanical oscillations of aphonograph needle to change its capacity correspondingl), said deviceforming one arm of said Vv'hcatstone bridge circuit, the adjacent 'armof said bridge comprising a balancing condenser,'two ratio arms alsoconsisting of condensers for completing said bridge circuit, whereby thefluctuations of capacity of said device vary the balance condition ofsaid bridge in accordance with said mechanical o:-:cillations, means toproduce superaudio frequency carrier current to be impressed on one ofthe diagonal branches of said bridge for being modulated in accordancewith said balance variations, and an output circuit including rectifyingmeans connected to the remaining diagonal branch of said bridge forutilizing said modulated currents.

8. Means for transforming mechanical oscillations into correspondingelectrical oscillations in combination, a Wheatstone bridge circuit, anelectrostatic device subjected to said mechanical oscillations to changeits capacity correspondingly, said device formin one arm of saidlVheatstone bridge circuit, the adjacent arm of said bridge comprising abalancing condenser, two ratio arms consisting of electrical impedancescompleting said bridge, whereby the fluctuations of capacity of saiddevice vary the balance condition of said bridge in accordance with saidmechanical oscillations, a space charge device having main electrodesand a plurality of coptrol electrodes, a circuit arrangement associatedwith the main electrodes and one of the control electrodes of said spacecharge device to produce undamped continuous oscillations, circuitconnections to impress said oscillations on one of the diagonal branchesof said bridge, further means to impress potential variations occurringat the ends of the remaining diagonal branch of said bridge due to saidbalance variations on the remaining control electrode of said spacecharge device, a utilization output circuit of said s ace charge deviceand means to cause electric currents varying in accordance with saidbalance variations to flow in said output clrcuit.

9. In combination, a Wheatstone bridge, an electric pick up device forphonographs and the like responsiveto the mechanical os' cillations ofthe phonograph needle, said device forming one arm of said Wheatstonebridge, the adjacent arm of said bridge comprising a balancingimpedance, two ratio ltd arms consisting of impedances completing saidbridge for maintaining said bridge normally balanced, said deviceunbalancing said bridge in accordance with the oscillations of saidneedle, a source of carrier current and means for modulating saidcarrier in accordance with the unbalance of said VVheatstone m bridgecircuit.

till] as the ends of the remaining diagonal branch til lid

tilt

of said bridge due to said balance variations. 11. In combination, aWheatstone bridge, an electric pick up device for phonographs and thelike responsive to the mechanical oscillations of the phonograph needle.said device forming one arm of said Wheatstone bridge, the adjacent armof said bridge comprising a balancing impedance, two ratio armsconsisting of impedances completing said bridge for maintaining saidbridge normally balanced, said device unbalancing said bridge, inaccordance with the oscillations of said needle, a space charge 'devicehaving main electrodes and a plurality of control electrodes, a circuitarrangement associated with the main electrodes and one of the controlelectrodes of said space charge device to produce undamped continuousoscillations, circuit connections to impress said oscillations on oneofthe diagonal branches of said bridge, further means to impresspotential variations occurring at the ends of the remaining diagonalbranch of said bridge due to said balance variations on the remainingcontrol electrode of said space charge device, a utilization outputcircuit of said space charge device and means to cause electric currentsvarying in accordance with said balance variations to flow in saidoutput circuit.

12. In combination, a Wheatstone bridge,

an electrostatic pick up device for phonographs and the like, comprisingin combination an electrostatic device adapted to be subjected tomechanical oscillations of a phonograph needle to change its capacitycorrespondingly, said device forming one arm of said Wheatstone bridgecircuit arrangement, the adjacent arm of said bridge comprising abalancing condenser, two ratio arms consisting also of condenserscompleting said bridge, whereby fluctuations of capacity of said devicevary the balance condition of said bridge in accordance with theoscillations of said phonograph needle, a space charge device havingmain electrodes and at least two control electrodes, a circuit arranement associated with said space charge devlce and one of the controlelectrodes to produce continuous oscillations, circuit connections toimpress said oscillations upon one of the diagonal branches of saidbridge circuit, a further means to apply potential variations occurringat the ends of the remaining diagonal branch of said bridge due to saidbalance variations, upon the remaining control electrode of said spacecharge device, and means to utilize electrical current variationscorresponding to said balance variations occurring in an output circuitof said space charge device.

13. In combination, an electrostatic pick up device for phonographs,comprising in combination an electrostatic 'device adapted to besubjected to mechanical oscillations of a phonograph needle to changeits capacity correspondingly, a space charge device having mainelectrodes and a plurality of control electrodes, circuit arrangementsassociated with said main electrodes and one of said control electrodesfor producing undamped continuous oscillations, means for modulatingsaid oscillations in accordance with the oscillations of saidelectrostatic device, and means to impress said modulated waves acrossanother of said control electrodes of said space charge device, autilization output circuit of said space charge device and means tocause electric currents varying in accordance with said modulations toflow in said output circuit.

14. In combination with an electric pick up system for phonographscomprising an electrostatic device adapted to mechanical oscillations ofthe phonograph needle to change its capacity correspondingly, means forgenerating a high frequency current, a balanced bridge, means forapplying said high frea quency across said bridge, means including saidelectrostatic device for modulating the high frequency current appliedacross said bridge in accordance with the amplitude of oscillation ofsaid electrostatic device, means for amplifying said modulatedoscillations, means for demodulating said modulated oscillations and aloud speaking device in operative relation with said last means foroperation in accordance with said demodulated oscillations.

15, -In combination with an electric ick up system for honographs, themetho of signalling whic comprises vibrating a pick up device inaccordance with the record made on the phonograph, varlyin a capacity inaccordance with the amp itu%e vibrations of the pick up device, applyinga carrier current across a balanced Wheatstone bridge, modulating thecarrier current in accordance with the varying capacity amplifying themodulated carrier current, de'modulating the carrier current andoperatin a loud speaker device in accordance with t e-demodulatedcarrier currents.

In testimony whereof he has aflixed his signature.

HUMFREY ANDREWES.

